In recent years, the generation of nitrogen oxides (NO.sub.x) by internal combustion engines has been the focus of increasing government regulation. NO.sub.x are believed to contribute to tropospheric ozone, a health hazard, and to also undergo a process known as photochemical smog formation in the presence of sunlight and hydrocarbons. NO.sub.x significantly contribute to the formation of acid rain and have been implicated as contributing to the undesirable warming of the atmosphere or greenhouse effect. It is anticipated that allowable NO.sub.x emissions for motor vehicles will be significantly decreased in the coming years.
Unfortunately, however, attempts to reduce NO.sub.x emissions in internal combustion engines, particularly those using lean air/fuel mixtures such as diesel engines, have generally resulted in only inadequate NO.sub.x reduction with a concurrent increase in particulate emissions. While NO.sub.x emissions in the exhaust streams of spark ignition engines (S.I.) have been somewhat reduced with the use of standard redox catalysts, the lean exhaust streams inherent in diesel engines have precluded such use. Lean air/fuel mixtures are herein defined as containing more air than fuel while a rich mixture contains less air.
Diesel engines in particular have posed a significant problem. They have the disadvantage of producing emissions with significant concentrations of NO.sub.x and particulates. The latter are generally manifested in the form of a black exhaust smoke or soot. Diesel engines also have a reputation for exhibiting significant noise and vibration during operation.
The control of automotive NO.sub.x emissions has traditionally been dependent on the use of three way catalysts (TWC), exhaust gas recirculation (EGR), the use of very fuel-rich or "reducing" engine exhaust conditions and/or retardation of injection timing.
Unfortunately, however, the use of three way catalysts often requires expensive and easily damaged exhaust oxygen sensors. Precise control and maintenance of the air/fuel ratio in a narrow band in the vicinity of stoichiometry is necessary. Also, as indicated above, the lean operating conditions of diesel engines and lean burn S.I. engines preclude the use of three way catalysts and reduction catalysts as cost effective methods for reducing NO.sub.x to the levels mandated by current and future government regulation.
Although present emission control systems incorporate EGR, its use has several disadvantages. Excessive use of EGR can cause rough combustion. Fouling of the injectors can result in loss of fuel economy due to a reduction in the combustion burn rate as well as a potential increase in hydrocarbon emissions.
Finally, although retarding the fuel injection timing can reduce NO.sub.x emissions, the amount of reduction that can practically be achieved is limited. This method of controlling NO.sub.x also has the significant disadvantage of reducing fuel economy.
The present invention resolves these deficiencies of the prior art by providing a NO.sub.x emission control system. The invention provides a method and apparatus wherein a selective reducing agent is introduced into the interior of a piston cylinder assembly, i.e. combustion chamber, of an internal combustion engine. The selective reducing agent is incorporated into the fuel feed stream and/or the air feed streams which combine to form the air/fuel mixture combusted in the engine. A characteristic of the invention is that the amount of selective reducing agent introduced into the combustion chamber is a function of the engine load. Upon the combustion of the air/fuel mixture containing the reducing agent, the treatment agent decomposes to react with one or more products of combustion to produce an engine exhaust stream having a reduced concentration of NO.sub.x.
Although the invention is particularly advantageous when used in compression ignition engines utilizing lean air-fuel mixtures, it may also be incorporated in engines having rich operating conditions such as traditional spark ignition engines.
Although the prior art has disclosed the use of selective reducing agents as a method of controlling NO.sub.x in combustion exhaust streams, no method or apparatus has been disclosed for use in automotive systems which incorporates the agent in the fuel and/or air feed streams entering the combustion chamber.
Unlike the instant invention, none of the prior art has achieved desired reductions of from 50 to 80% in NO.sub.x concentrations using overall ratios of NO.sub.x to selective reducing agent of 2:1 to as low as 10:1. It has been found that in the instant invention, the amount of reducing agent required is a function of when in the operating cycle the agent is introduced into the combustion chamber and how much NO.sub.x is generated during that portion of the cycle.
Prior art patents disclosing the use of cyanuric acid to reduce NO.sub.x emissions such as U.S. Pat. Nos. 4,731,231; 4,800,068; 4,886,650; and 4,908,193 to Perry, disclose the incorporation of cyanuric acid into an exhaust stream post-combustion. These prior art patents teach that large amounts of solid cyanuric acid are required to achieve desirable automotive exhaust streams. In addition, these patents fail to provide practical, efficient and cost effective methods of using cyanuric acid in consumer vehicles. According to the teachings of these patents, such systems will preferably be designed to utilize stoichiometric amounts of NO.sub.x and HNCO where excesses of either NO.sub.x or HNCO are used. Acceptable levels of excess NO.sub.x and HNCO are taught to be "in the range of about 1.01 to about 1.1 or more on a stoichiometric basis". Finally, the overall disclosures of these prior art patents teach away from the use of cyanuric acid in oxygen rich environments.
U.S. Pat. No. 4,861,567 to Heap et al. expressly teaches that the decomposition of cyanuric acid must take place in an "oxygen free (fuel rich) environment".
As a result, such prior art patents fail to provide a method whereby the NO.sub.x emissions of internal combustion engines in general, and, those of diesel engines and lean burn spark ignition engines in particular, can be reduced with the use of selective reducing agents in a manner which is both practical, highly effective and economical.